Plastics

The term 'plastic' is ambiguous. Literally, it means a material that is malleable (can be shaped without breaking), and can be engineered and molded by flow (the raw material is liquid during manufacture). However, 'plastic surgery' does not refer to the use of materials, but rather emphasises the malleable aspect of plastic, in the reshaping of flesh.

The general use of the word 'plastic' is for a broad range of materials. These consist of large, long-chain molecules, which are organic polymers, and are usually synthetic, with high molecular mass. The large range of plastics have different properties, such as hardness, malleability, elasticity, resistance to temperature and chemicals, which can be controlled by the manufacturing process, and the additives. There are three main sub-groups of plastics: thermoplastics, duroplasts, and elastomers. The raw substance from which plastic is made is typically petrochemical in origin, although biosynthetic plastics can be derived from non-fossil sources, such as natural plant oils.

In 1844, Charles Goodyear finalised his technique for vulcanizing rubber, and industry embarked on a revolution in materials made to order.

The 'Miracle' of Plastic

Celluloid was discovered as a result of a competition to find a substitute for ivory for billiard balls, and made movie films possible

In 1856, the metallurgist Alexander Parkes produced Parkesine. Prompted by a competition to find a substitute for ivory for billiard balls, he had treated cellulose with nitric acid to produce cellulose nitrate (pyroxilin), which, when dissolved in alcohol, hardened into a transparent elastic material. It was pigmented so that it would resemble ivory. It was registered as Celluloid in 1870, and, being a thermoplastic, could be moulded to any shape, from dentures to movie camera film.

However, it was not until 1907 that the first fully synthetic plastic was commercially produced. Leo Baekeland invented the term 'plastics' for his new 'bakelite'. Once the advantages of plastics over traditional materials, such as wood, stone, leather, metal, glass and ceramics, for household and other goods, had been realised, the plastics industry leapt upon every new polymer the chemists could produce.

Bakelite was the first commercial fully synthetic plastic, invented by Leo Baekeland in 1907

Development in the 1920s and 1930s led to synthetics with enormous commercial potential, not only as a cheaper alternative to traditional materials, but materials, such as nylon, which made entirely new products possible. World War Two provided a stimulus for the mass-production of plastics, such as acrylic glass as an alternative to silicate glass for airplane windscreens.

Leo Baekeland, 1863 - 1944

By the 1950s, mass production of products made from plastics like polystyrene (PS) and polyvinyl chloride (PVC) transformed the consumer manufacturing industries. By the 1980s, the world was producing trillions of plastic bags for throw-away shopping conveniences, and packaging still accounts for as much as 40% of plastic use.

Plastic is a durable problem, and our legacy for future generations

As a result, the love affair with the new materials grew into a nightmare, as millions of tonnes of discarded plastic waste entered the biosphere. Its characteristic of durability and non-degradability, so treasured by the manufacturing industry, was a cause of enormous concern for the environment. The natural world does not know what to do with unnatural products.

Reclaiming value from plastic waste is a very important issue in modern waste management. Up to 20% of plastic by mass can be additives to modify the properties of the basic carbon polymer chain. These additives, such as highly toxic organotin, create complications for effective waste management.

Plastics Recycling

Plastic is cheap to produce, light and easily stored. It makes airtight, hygienic and tough containers, and strong, lightweight carriers, so is ideal for packaging, storage and transport. Plastic production uses 8% of the world oil production, which is 7 million barrels a day. Half of this oil is for feedstock in the production of plastic, and half is consumed in the production process itself. Reducing the amount of plastic produced will reduce energy consumption and emissions of carbon-dioxide (CO2), nitrogen-oxide (NO) and sulphur-dioxide (SO2) during manufacture, and reduce non-biodegradable landfill waste.

There are three basic ways to ensure the most is obtained out of plastic:

Re-use

The low cost of plastic has a negative effect on rates of recycling and reuse. Thankfully, the concept of reusing bags for shopping trips is changing the throw-away, single-use habit which has caused environmental contamination for decades.

Similarly, transport crates, storage vessels, and many other plastic items can be reused by industry and commerce, and regulations are being imposed to ensure that economic criteria are not the only factors considered by management. Environmental movements activate consumer power in encouraging, for example, customers to return packaging to the retail outlets, bringing pressure back up the supply line for a more efficient policy which reduces wastage.

Recycling

Recycling does not come for free

The first challenge in recycling plastics is the difficulty of post-use separated collection from the consumer. Ferrous and aluminium cans can be collected together, and easily separated by magnets. However, the public are not sensitive to all the groups of plastics, and low collection rates of recyclable plastic are still the norm. Plastic waste is still widely sorted by hand into polymer category and colour, in the collection or reprocessing facilities. Some technologies are being developed to aid sorting. These include X-ray fluoresence, infrared and near-infrared spectroscopy, electrostatics and flotation.

The second hurdle relates to the practicality of cleaning, removal of contaminants (such as paper labels and glue residues), colour mixes, and the economicability of options for the reclaimed material. Mechanical recycling involves melting, shredding or granulation. The sorted plastic is melted down, as it is or after shredding into flakes, and regranulated.

Feedstock recycling involves a process of breaking the polymer into its constituent monomers, which are then used in applications in refineries, petrochemical and chemical industries. The technologies being developed for this purpose include pyrolysis, hydrogenation, gasification, and thermal cracking. The advantage of feedstock over mechanical recycling is greater impurity tolerance. Disadvantages are that it is capital intensive, and scale, whereby very large quantities (in excess of 50 kt p.a.) are required to make the operation economic. One tonne of plastics is equivalent to 20,000 two litre drinks bottles or 120,000 carrier bags.

Plastic can be recycled for use in products as diverse as polyethylene bin liners and carrier bags, piping made of PVC, building elements such as flooring, window frames, and insulating board, garden furniture, clothing (e.g. fleeces), and office accessories.

Energy Reclamation from plastic waste

Waste-to-Energy waste incinerator, Amsterdam

The first principle is the avoidance of waste generation in the first place. The second is where possible the reuse or recycling of substances, which are separated from the regular waste stream. For the remaining amount, the better option is exploitation of the energy content, and only then should any material that cannot be reused, recycled or which has no energy value, should be carefully landfilled under controlled conditions.

Plastic can be incinerated in waste-to-energy plants, and the energy used to generate electricity. The flue gases need to be scrubbed, as they contain dioxins and other pollutants.

Degradable plastics

It is unlikely that plastic will ever be 100% recycled or reclaimed, at least not in the foreseeable short-term. Therefore, degradability of plastic is the next best option, ensuring that if the plastic ends up in a landfill, or worse, in the environment, it will degrade on a timescale which reduces the danger to wildlife, and nuisance to humans, considerably.

Many retailers now use degradable plastic carrier bags. Bio-degradable plastics contain a small percentage of non oil-based substance, such as corn starch, which will allow bacteria and ageing to break up the polymers. Fastfood cutlery made of this degradable polymer can now be composted without segregation.

Plastic is a great hazard for wildlife

Photodegradable plastics are sensitive to sunlight, and will break up when exposed for a predetermined and controllable time period. An example of a needed application is the deadly beer 'six-pack' carriers, which are strong enough for the purpose they are designed for, but which will photodegrade after 6 weeks, safeguarding wildlife, particularly marine creatures, which easily get trapped in the circular rings.

Genetically engineered bacteria can synthesis a biodegradable plastic, and additives aid the rate of biodegradation.

There are three problems with degradable plastic: 1. the plastic needs particular conditions to successfully degrade. If photodegradable plastics are dumped in a landfill, away from light, they will not decompose. 2. Biodegradble waste will produce emissions of CO2, or worse, methane, if it degrades anaerobically (without sufficient oxygen). 3. Consumers may reverse the better trend of waste minimisation if they believe the waste is no longer an environmental hazard.

Bio-Plastics

Bio-plastics are synthetic polymers made from plants, sugars, or plastic which is grown within genetically modified plants and micro-organisms. The use of bio-plastic avoids the use of non-renewable fossil fuel as feedstock, and also reduces risks from chemical additives in standard plastics. Toy manufacturers have begun to switch over to bio-plastics for health and safety reasons.

UK plastics waste policy

In 2001, the Environment Agency reported that 80% of post-use plastic waste was sent to landfill, 8% was incinerated and 7% recycled. These figures do not include reprocessing of residual polymers after industrial plastic production, of which 95% is used in further production.

The UK has an incomplete system of plastic waste collection, with 4,000 plastic bottle collection banks collecting 24 ktonnes of plastic bottles, which is 5.5% of the total in circulation.

Type

Name

Formula

Applications

Recycling

Elimination

PE

Polyethylene

(C2H4)n

supermarket bags, plastic bottles, and many other packagings and uses

Not recycled, but PE can be made from renewable feedstock, such as wheat grain and sugar beet

Not readily biodegradable, a limited amount can be degraded by bacteria, and research into turning PE into oil is underway

LDPE

Low-density polyethylene

(C2H4)n

supermarket carrier bags, bin liners

Not recycled, but PE can be made from renewable feedstock , such as wheat grain and sugar beet

Not readily biodegradable, a limited amount can be degraded by bacteria, and research into turning PE into oil is underway